Quantitative Backdraft Experiments
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Quantitative Backdraft Experiments C. M. FLEISCHMANN University of Canterbury Chr~stchurch,New Zealand P. J. PAGNIand R. B. WILLIAMSON Un~vers~tyof Californ~a,Berkeley Berkeley. CA 94720. USA ABSTRACT This paper focuses on 17 experiments in a 1.2 m by 1.2 m by 2.4 m compartment. A methane burner, flowing at either 70 kW or 200 kW, was ignited inside a closed compartment and burned until the initially available oxygen was consumed. After the fire self-extinguished, the burner was left on allowing the unburned fuel mass fraction in the compartment to increase. After removing a hatch, covering a 1.1 m wide by 0.4 m high slot opening, a gravity current entered the compartment. It traveled across the floor, mixed with the unburned fuel, and was ignited by a spark near the burner. After mixture ignition, a backdraft occurred as a deflagration ripped through the compartment culminating in a large external fireball. Histories recorded prior to backdrafl included: fuel flow rates, upper layer temperatures, lower layer temperatures, upper layer species concentrations for 02, CO2, CO, and HC. Data collected to quantify the backdraft included opening gas flow velocities and compartment pressures. Results indicate that unburned fuel mass fractions >lo% are necessary for a backdrafl to occur. KEYWORDS: compartment fires, backdraft, fire initiation, explosion hazards INTRODUCTION The dangerous consequences of a backdraft are documented in numerous fire service publications and training manuals.' ,2,3,4 However, little research has been done in the area of backdrafts, and only recently has a scenario been presented describing the fundamental physics underlying backdraft phenomena5. A backdraft is defined as a rapid deflagration following the introduction of oxygen into a compartment filled with accumulated unburned fuel. The scenario presented here assumes a fire in a closed room. The fire heats up the room, and leakage in the bounding surfaces minimizes the pressure differential. The hot layer descends FIRE SAFETY SCIENCE-PROCEEDINGS OF THE FOURTH INTERNATIONAL SYMPOSIUM, pp. 337-348 337 Copyright © International Association for Fire Safety Science over the fire as the oxygen concentration is reduced and the combustion efficiency decreases. Excess pyrolyzates accumulate in the upper layer forming a fuel rich mixture of low oxygen content. A small flame or glowing ember exists as a source of ignition. Suddenly, a new ventilation opening is provided and cold, oxygen rich, air enters the compartment and propagates across the floor as a gravity current6. Large scale mixing in the gravity current provides mixed zones within the flammable range that ignite when they contact an ignition source. Once ignited, a flame propagates through the compartment and drives the remaining unburned fuel out through the opening to burn outside the compartment in a spectacular fireball. In this paper, experimental results are presented from a series of half scale experiments attempting to quantify backdraft. Experimental variables included fuel flow rate, burn time, ignition location, ignition delay time, burner height, species sample location, and opening size7. This paper focuses on 17 experiments in which the opening geometry was a vertically centered, horizontal slot, in one end wall; the fuel source was a 0.30 m square burner, 0.30 m above the floor; and the ignition source was a spark located opposite the opening. Two different burner flow rates were used, 70 kW and 200 kW. Data collected in these experiments had two goals: 1) to characterize the conditions in the compartment prior to backdraft and 2) to quantify the severity of the deflagration. EXPElUMENTAL DESIGN Apparatus: Experiments were conducted in a special compartment designed to safely control the dangerous overpressures expected in backdrafts. The experimental apparatus dimensions were limited to half a small residential room to minimize the expected hazard and to allow the experiments to be conducted inside a 900 m3 facility. Figure 1 shows a schematic of the apparatus giving the internal dimensions of the compartment and the locations of the instrumentation. In order to control the overpressure hazard, one long wall was a pressure relief panel designed according to refs. 8 and 9 . The interior surfaces of the compartment were lined with a 50 mm thick refractory fiber blanket installed over the gypsum wallboard on the walls and ceiling to provide the primary thermal resistance for the structure. A 0.9 m high by 1.5 m wide observation window of Neoceram1° was installed in the wall opposite the pressure relief panel. To simulate a window or door, a 0.4 m high by 1.1 m wide opening was centered in the short wall opposite the burner, see Fig. 1. This opening was covered with a computer activated hatch which was opened after the fire had been burning for several minutes. A methane burner, 0.3 m square and 0.3 m high, was used in all of these experiments. The burner was placed against the wall opposite the opening, as seen in Fig. 1. A pilot flame was used to ignite the burner and was turned off 10 s after the start of the experiment. The primary ignition source for the backdraft was a spark ignitor located 0.45 m above the floor and centered over the top of the burner. For three of the experiments, the primary spark malfunctioned and the backup spark ignitor used to ignite the pilot light had to be used to ignite the backdraft. The backup spark was located 0.35 m above the floor and centered on the side of the burner facing the opening. A 10,000 volt transformer was used for each spark ignitor to produce an arc between two 3 mm diameter 308 stainless steel electrodes 5 mm apart. Pressure Relief FIGURE 1 Sketch of the half scale backdraft compartment showing important features of the apparatus. Every effort was made to seal all construction holes to control leakage. The primary sources of leakage into the compartment were found to be around the pressure relief panel and the opening hatch. A small 0.1 m diameter pressure relief vent was placed at the floor level to relieve the pressure from the initial burner ignition. A computer controlled cover closed over this vent 15 s after ignition. Additional details of the apparatus can be found in refs. 5 and 7. Species Concentration: In order to characterize the compartment conditions prior to a backdraft, the species concentration histories in the upper layer were recorded. Gas concentrations measured were: oxygen (O,), carbon dioxide (CO,), carbon monoxide (CO), and total hydrocarbons (HC). Continuous gas samples were taken with stainless steel probes located 0.6 m from the opening wall, 0.2 m from the ceiling, and 0.6 m from the side wall. The hydrocarbon sample required a separate heated sample line to prevent the loss of hydrocarbons due to condensation. A flame ionization detector was used to measure the hydrocarbon (methane) concentration. The expected hydrocarbon range was outside the limits of the analyzer and required dilution. Details of the dilution system and species balances performed can be found in Ref. 7. Temperatures: A vertical thermocouple tree was placed 0.6 m from the opening wall and 0.2 m from the pressure relief panel, as shown in Fig. 1. The thermocouples were made from 0.5 mm type K thermocouple wire with a stainless steel overbraid. The average bead diameter was 1.1 mm. The ten thermocouples were located at 0.10 m intervals, with the highest thermocouple at 0.15 m below the ceiling. The temperatures reported here are uncorrected values. The thermal interface height history was calculated from the time dependent temperature profiles recorded from the thermocouple tree. The profiles were converted into unsteady average upper and lower layer temperatures using the method Quintiere et. al." applied to steady state temperature profiles. Compartment Pressure: The compartment pressure history was recorded using an electronic pressure transducer with an effective range of 0 to 1250 Pa. The pressure port was mounted in the stationary wall opposite the pressure relief panel at floor level. The ambient pressure reference was taken outside the building. Hatch Flow: The flow in and out of the compartment after the hatch was opened was recorded using six bidirectional probes in the h/3 centered slot. The probes were located in the horizontal center of the opening, 65 mrn apart, with the outer probes 43 mm from the soffit and sill. Probe velocities were calculated using the relationship given by McCaffrey and Heskestad.I2 The mass flows were obtained by integrating the velocity and density profiles over the height of the opening. Data Acquisition System: Data from each sensor was recorded using a HP VECTRA 80486-33 computer with an. 8 channel multifunction analog and digital inputloutput board. Two 32 channel analog input muliplexors were connected to this system. A total of 3 1 thermocouple and 17 voltage channels were measured. The system was capable of recording each channel 50 times a second. For experiments greater than 600 s the data was collected at a rate of 10 scads until 20s before opening when the rate was automatically increased to 50 scads. The reduced scan rate for the initial period of the experiment was necessary to reduce the data file size. PROCEDURE Before each experiment a 60 s baseline was taken to record the initial conditions. A pilot flame was ignited at the burner 5 s before the start of the experiment. At 0 s a solenoid was opened on the methane flow to the burner and the clock was reset to zero. The burner was left on for a predetermined time period. Gas flow to the burner was terminated 5 s before the hatch was opened.